The hard problem of consciousness (Chalmers 1995) is the problem of explaining the relationship between physical phenomena, such as brain processes, and experience (i.e., phenomenal consciousness, or mental states/events with phenomenal qualities or qualia). Why are physical processes ever accompanied by experience? And why does a given physical process generate the specific experience it does—why an experience of red rather than green, for example?

Hard problems and easy problems

The hard problem contrasts with so-called easy problems, such as explaining how the brain integrates information, categorizes and discriminates environmental stimuli, or focuses attention. Such phenomena are functionally definable. That is, roughly put, they are definable in terms of what they allow a subject to do. So, for example, if mechanisms that explain how the brain integrates information are discovered, then the first of the easy problems listed would be solved. The same point applies to all other easy problems: they concern specifying mechanisms that explain how functions are performed. For the easy problems, once the relevant mechanisms are well understood, there is little or no explanatory work left to do.

Experience does not seem to fit this explanatory model (though some reductionists argue that, on reflection, it does; see the section on reductionism below). Although experience is associated with a variety of functions, explaining how those functions are performed would still seem to leave important questions unanswered. We would still want to know why their performance is accompanied by experience, and why this or that kind of experience rather than another kind. So, for example, even when we find something that plays the causal role of pain, e.g. something that is caused by nerve stimulation and that causes recoil and avoidance, we can still ask why the particular experience of hurting, as opposed to, say, itching, is associated with that role. Such problems are hard problems.

Cognitive models of consciousness (Barrs 1988) are sometimes described as potential solutions to the hard problem. However, it is unclear that any such model could achieve that goal. For example, consider global workspace theory, according to which the contents of consciousness are globally available for various cognitive processes such as attention, memory, and verbal report. Even if this theory is correct, the connection between such processes and experience—e.g., why they are accompanied by experience at all—might well remain opaque. For similar reasons, discovering neural correlates of consciousness might leave the hard problem unsolved: the question as to why those correlations exist would remain unanswered. Nevertheless, scientific advances on cognitive models and neural correlates of consciousness might well play important roles in a comprehensive solution.

Relation to arguments against physicalism and the explanatory gap

The hard problem is often discussed in connection to arguments against physicalism (or materialism) which holds that consciousness is itself a physical phenomenon with solely physical properties. One of these arguments is the knowledge argument (Jackson 1982), which is based on thought experiments such as the following. Mary is a super-scientist with limitless logical acumen, who is raised far in the future in an entirely black-and-white room. By watching science lectures on black-and-white television, she learns the complete physical truth—everything in completed physics, chemistry, neuroscience, etc. Then she leaves the room and experiences color for the first time. It seems intuitively clear that upon leaving the room she learns new truths about what it is like to see in color. Advocates of the knowledge argument take that result to indicate that there are truths about consciousness that cannot be deduced from the complete physical truth. It is inferred from that premise that the physical truth fails to completely determine the truth about consciousness. And the latter result, most agree, would undermine physicalism.

The hard problem relates closely to the claim that Mary learns new truths about color experiences when she first has such experiences. Arguably, if she learns new truths at that time, this is because the nature of color experiences cannot be fully explained in purely physical terms; otherwise, the reasoning runs, she would have already known the relevant truths. If such experiences are fully explicable in physical terms, then they should be objectively comprehensible, and Mary seems well positioned to grasp all objectively comprehensible properties. The general idea here is sometimes expressed as the claim that there is an explanatory gap (Levine 1983) between the physical and the phenomenal.

A second argument often associated with the hard problem is the conceivability argument (Kripke 1972, Chalmers 1996). According to one version of the conceivability argument, also called the zombie argument, one can conceive of a micro-physical duplicate of a human that lacks conscious experiences. Given this, it is argued, such a micro-physical duplicate is possible, which entails that the physical facts do not necessitate the phenomenal or experiential facts. This, according to most philosophers, indicates that physicalism is false.

While many philosophers doubt that the conceivability of these zombie duplicates is indicative of their possibility, the hard problem primarily concerns the first step of the argument. If we can conceive of micro-physical duplicates of ourselves that lack consciousness, then we lack a complete explanation for why the physical facts give rise to the experiential or phenomenal facts. This again shows the existence of an explanatory gap.

Reductionism

There is no consensus about the status of the explanatory gap. Reductionists deny that the gap exists. They argue that the hard problem reduces to a combination of easy problems or derives from misconceptions about the nature of consciousness. For example, Daniel Dennett (2005) argues that, on reflection, consciousness is functionally definable. On his view, once the easy problems are solved, there will be nothing about consciousness and the physical left to explain.

Reductionists often appeal to analogies from the history of science. These philosophers compare nonreductionists, who accept the existence of the explanatory gap, to 17th Century vitalists concerned about the hard problem of life. Comparisons are also made to the scientifically ignorant concerned about hard problems of heat or light (Churchland 1996). Science has shown that the latter concerns are overblown: life, heat, and light can be physically explained. Likewise, say reductionists, for consciousness.

Nonreductionists usually reject such analogies. Part of the analogy is usually accepted: the vitalists doubted that how organisms reproduce, move, self-organize, etc., could be explained in purely physical terms, in much the same way that nonreductionists doubt that consciousness can be explained in purely physical terms. However, what the vitalists sought to explain was how certain functions are performed. By contrast, consciousness does not seem to consist in the performance of functions. Nonreductionists take that difference to undermine the analogy between the hard problem of consciousness and the alleged hard problem of life. They reject the reductionists’ other analogies on similar grounds.

Reductionism is entailed by influential theories in the philosophy of mind, including philosophical behaviorism, analytic functionalism, and eliminative materialism. Some philosophers take the merits of those positions, such as their relative parsimony, to provide grounds for a reductionist approach to the hard problem. Other philosophers accept the existence of the explanatory gap and thus regard the hard problem as evidence against those theories.

Nonreductionism

All nonreductionists believe that the explanatory gap is genuine, but some nonreductionists argue that the gap is compatible with physicalism (Loar 1990/97). For nonreductionist physicalists, the gap reflects something about our perspective on the world, not the world itself. These philosophers hold that consciousness is an entirely physical phenomenon, and thus that phenomenal truths are nothing over and above physical truths, even though phenomenal truths cannot be deduced from micro-physical truths or the sorts of truths that Mary learns from her lectures.

Non-reductionists must explain how to reconcile physicalism with the explanatory gap. (Reductionists do not share this burden, since they reject the gap.) Here nonreductionists sometimes invoke analogies to Kripkean (1972) empirical necessities. According to Kripke, the fact that heat is (decoherent) molecular motion is absolutely necessary—there is no possible situation in which there is one without the other—even though that fact was discovered empirically. One might object on the grounds that we can easily imagine a situation in which there is heat but, it turns out, no molecular motion. Against this, Kripke argues that on reflection such a situation is inconceivable. What we imagine existing without molecular motion is the sensation of heat—an experience typically caused in us by molecular motion—and not heat itself. Non-reductionists sometimes argue that similar reasoning could be used to explain why, in spite of the explanatory gap, the physical truth necessitates the truth about consciousness. However, as Kripke himself argues, in the case of consciousness there does not appear to be a distinction corresponding to that between heat and the sensation of heat. For example, anything that feels like pain is ipso facto pain. So, Kripke’s reasoning does not straightforwardly extend to the empirical necessities entailed by nonreductionist physicalism.

Many nonreductionists acknowledge that more is required to reconcile physicalism with the explanatory gap. Here it is common to appeal to distinctive features of phenomenal concepts. Some propose that phenomenal concepts are distinctive in that their referents—phenomenal states—are constituents of those very concepts. For example, David Papineau (2002) suggests that phenomenal concepts have the form that state: —, where the blank is filled in by an embedded phenomenal state, in something like the way a word may be embedded within quotation marks. He argues that the quotational structure of phenomenal concepts will produce a distinctive phenomenal/physical epistemic gap even if the embedded state is physical. But whether any such proposal can meet the nonreductionist’s burden remains controversial (Chalmers 2007).

Some nonreductionists take the hard problem as a reason to reject physicalism. On most nonphysicalist views, consciousness is regarded as an irreducible component of nature. These views tend to differ primarily on how they characterize the causal relationship between consciousness and the physical world. According to interactionist dualism, for example, consciousness has both physical causes and physical effects; according to epiphenomenalism consciousness has physical causes but no physical effects; and according to neutral monism phenomenal properties are the categorical bases of physical properties, which are dispositional (neutral monism might or might not count as a version of physicalism, depending on whether the categorical bases physical properties are considered physical).

Psychophysical theories

Some believe that solving the hard problem will require constructing a psychophysical theory that includes fundamental laws. No such theory has been developed in great detail, but some speculative proposals have been advanced. Certain interactionist dualists argue that phenomenal properties affect brain processes by filling in gaps resulting from quantum indeterminacy (Eccles 1986). Theories emerging from that sort of argument may involve positing psychophysical laws. And David Chalmers (1995), a leading nonreductionist, tentatively proposes that the basic link between the phenomenal and the physical exists at the level of information. He formulates a double aspect principle, on which phenomenal states realize informational states that are also realized in physical, cognitive systems such as the brain. Either proposal might provide a kind of solution to the hard problem: the laws would enable deductions of specific instances of experience from underlying physical structures. An important vestige of the hard problem would, of course, remain: there would still be the question as to why these psycho-physical laws existed and not others. Such theorists are likely to argue that these laws are primitive, just like the basic laws of physics, and so the vestigial hard problem is neither more nor less puzzling than the question as to why the physical constants are what they are.

Reductionists will argue that such proposal are misconceived, either because they depend on confused notions of consciousness or because they presuppose that solutions to the easy problems will not yield a solution to the hard problem. Nonreductionist physicalists will reject those reductionist arguments, but they also tend to reject the need for a fundamental psychophysical theory. Not all such theories conflict with nonreductionist physicalism. Indeed, these philosophers might accept something like Chalmers’ proposal and regard it as a way to bridge the explanatory gap. Unlike Chalmers, however, they will regard phenomenal information as a special sort of physical information—special in that its connection to other sorts of physical information will remain opaque without appropriate psychophysical laws.

Mysterianism

Some argue that we are unable to solve the hard problem. This view is sometimes called mysterianism, and its best-known champion is Colin McGinn (1989). McGinn argues that our minds are simply not constructed to solve the hard problem; we are cognitively closed to it, in something like the way rats are cognitively closed to calculus problems. But unlike the rats, we can grasp the nature of the problem that, according to McGinn, we cannot solve.

McGinn locates the source of our cognitive closure not in the hard problem’s intrinsic complexity—he allows that the solution may be simple—but rather in how we form theoretical concepts. In his view, we form such concepts by extending concepts associated with perception of macroscopic objects. And he argues that any concepts produced by this mechanism will, like familiar physical concepts, inevitably leave the hard problem unsolved. This argument—both the premise about concept formation and the mysterian inference—is controversial (Stoljar 2006). And there are versions of mysterianism that do not rely on the argument. These include less pessimistic versions on which scientific advances may one day enable us to solve the hard problem (Nagel 1998, Stoljar 2006).

Mysterians differ on both reductionism and physicalism. McGinn and Thomas Nagel, a less pessimistic mysterian, reject reductionism. Daniel Stoljar, another less pessimistic mysterian, is officially neutral on reductionism. And whereas McGinn and Nagel are officially neutral on physicalism, Stoljar accepts it; indeed, his defense of mysterianism assumes physicalism.